A commonly available bird poison is thought to be behind the deaths of birds at the harbour lookout on State Highway 1 just south of Whangarei.
The carcasses of three pukeko and some sparrows, along with poison-laced vegetable scraps strewn around the roadside reserve, are evidence of the latest poisoning.
The vegetable scraps contain the poison's trademark pink dye.
Earlier this year, the reserve's resident flock of chickens was wiped out, possibly with the same product, thought to be Alphachloralose.
Alphachloralose is mainly used by market gardeners and other large-scale growers to kill birds, and can be purchased from stock and farm supply outlets without a poison licence.
The poison works by lowering the birds' body temperature until they lose consciousness and die. Under ideal circumstances, the birds would be collected and killed humanely. In less controlled circumstances, they just die where they fall.
SPCA inspector and education officer Rachael Wyatt said no one she had spoken to had admitted to laying the poison at the lookout.
"It could very well be Joe Blow from the public, but why would they?" she asked.
Ms Wyatt was called to the lookout on SH1 on Sunday after someone reported the dead birds.
She arrived in time to save a sick white rooster, the only poultry bird still living at the site. The rooster was showing typical signs of the poisoning - unable to walk and having seizures. Ms Wyatt caught the rooster, warmed it up until it became fully conscious and took it into SPCA care.
Ms Wyatt is concerned that with the deadly vegetable scraps still at the scene other birds, domestic animals or children might come into contact with the poison. "The uneaten toxic bait must be removed as it poses a threat," she said.
Motorists and other people tended to feed the chickens that hang out - usually after being dumped - on road reserves, and those food scraps in turn attract rats.